U.S. Pat. No. 3,615,376 describes a nickel base superalloy having a composition consisting essentially of, in weight %, 0.1 to 0.3% C, greater than 13% to less than 15.6% Cr, greater than 5% to less than 15% Co, 2.5% to 5% Mo, 3% to 6% W, 2% to 4% Al, 4% to 6% Ti, 0.005% to 0.02% B, up to 0.1% Zr, and balance essentially nickel with the ratio of Ti to Al being greater than 1 but less than 3; the sum of Ti and Al being 7.5%-9 weight %; and the sum of Mo and half of the W being 5 to 7 weight %. Carbon concentrations of 0.08 weight % and below are said to be insufficient to achieve high temperature alloy strength properties.
This nickel base superalloy exhibits improved high temperature stability, strength, and corrosion resistance. However, large gas turbine engine blades and vanes of industrial gas turbine (IGT) engines conventionally cast (e.g. equiaxed casting microstructure) from this superalloy exhibit inadequate machinability as a result of the cast microstructure containing large equiaxed grains, chemical segregation in thicker sections of the IGT castings, and undesirable carbide formation at the grain boundaries that embrittles the grain boundaries and can result in cracking or carbide/grain pull out during subsequent machining of the casting by such machining processes as grinding.
As a result, current machining practice for such IGT castings involves greatly increasing machining times by reducing machining feed rates to reduce cracking and carbide/grain pullout and produce a satisfactory machined surface finish. For example, the machining time of a large IGT equiaxed cast gas turbine engine blade cast from the above superalloy typically consumes 270 minutes.
An object of the present invention is to modify the above nickel base superalloy to unexpectedly and substantially improve its machinability, especially machinability of large equiaxed IGT castings produced from the modified superalloy, without adversely affecting the desirable alloy high temperature mechanical properties.